This invention relates to the dehydration of alpha-methylbenzyl alcohol (also known as alpha-phenylethanol or methyl phenyl carbinol) to produce styrene. The invention is more particularly concerned with the recovery of valuable monomeric compounds from residual materials produced in the separation of components of the dehydrated mixture.
An early process by which styrene was produced commercially for a short period of time comprised oxidation of ethylbenzene to acetophenone in the liquid phase, hydrogenation of the ketone to alpha-methylbenzyl alcohol, and catalytic dehydration of the alcohol through styrene. This process is described in Kirk-Othmer, "Encyclopedia of Chemical Technology," 1954, Volume 13, pp. 134-36. A more recent, commercial process for the dehydration of alpha-methylbenzyl alcohol is described on pp. 62-63 of the above-mentioned Volume 19 of the 2nd edition of the Kirk-Othmer text. The dehydration of ethylmethylbenzyl alcohol can be effected in any convenient manner, either in the liquid phase or in the vapor phase. See for example the methods described in U.S. Pat. No. 3,658,928. A particularly attractive process is the liquid phase dehydration process described in Becker et al. U.S. Pat. No. 3,526,674 issued Sept. 1, 1970, disclosure which patent is incorporated herein by reference.
According to the process described in the Becker patent, alpha-methylbenzyl alcohol is subjected to catalytic dehydration in the presence of a liquid-phase reaction medium at a temperature above about 200.degree. C., but below the decomposition temperature of the liquid phase reaction medium, in the presence of an appropriate catalyst. Ordinarily, the liquid-phase medium is provided by accumulating a sufficient quantity of the residual material which is formed during the dehydration reaction. These residual materials are retained in the dehydration system, except for a relatively small purge to prevent an undesired buildup. The residual material can be the sole liquid-phase reaction medium or it can be mixed with either polar or non-polar, relatively high boiling solvents of the type described in the Becker patent. During the course of the reaction, the alpha-methylbenzyl alcohol forms water and the desired styrene. Under the reaction conditions employed in the Becker process, the water and styrene are volatilized substantially as rapidly as they are formed and are therefore readily removed from the reaction zone. Alpha-methylbenzyl alcohol can also be volatilized as the reaction proceeds and is suitably condensed and returned to the reaction zone. The small amounts of high-boiling residual by-products remain essentially unvolatilized and are purged from the liquid-phase medium, periodically or continuously, when they accumulate to an undesired extent.
The purged stream which is thus withdrawn from the liquid-phase reaction medium ordinarily represents a very minor quantity and its withdrawl does not effect the commercial attractiveness of the process. However, treatment of this purge stream is known to yield additional quantities of styrene and styrene precursors, increasing the overall styrene selectivity of the process and reducing waste disposal problems.
In the case of vapor-phase operation, a residual by-product fraction may also be recovered from the vaporous effluent from the reactor and what has been said above regarding the residual purge from liquid-phase dehydration is also applicable to such vapor-phase residual fractions.
U.S. Pat. No. 4,273,622 discloses a process wherein the residual portion of the alpha-methylbenzyl alcohol dehydrate is distilled in two stages, the first stage being carried out at a temperature of 180.degree. C. to 275.degree. C. under vacuum, and the second stage treatment of the first stage residue being carried out at a higher temperature than the first stage temperature for a longer period of time. Preferred second stage temperatures are within the range from 300.degree. C. to 425.degree. C. Although second stage pressure is not disclosed to be critical, preferred pressures are at least 500 mm Hg and are at about atmospheric pressure.
Recovery of styrene monomer from polystyrene by thermal cracking at temperatures excess of 300.degree. C. is known. Common by-products from thermal cracking of polystyrene are toluene, ethylbenzene and alpha-methylstyrene.
Dehydration residues contain predominantly polymeric materials (largely polystyrenes) although minor amounts (about 2-10 wt. percent) of styrene monomer, acetophenone, alpha-methylbenzyl alcohol, beta-phenylethanol, and 1,4 dipropyl benzene may also be present. Polymeric materials include polymeric hydrocarbons, polymeric oxygenated materials and interpolymer or condensation products containing both hydrocarbon and oxygenated moieties.
When this residual material is treated directly at temperatures necessary to achieve the polymerization, the 4,273,622 patent notes the contained monomeric components react with polymeric components to form additional polymeric substances which result in a significantly more refractory polymeric mass. By employing the particular distillation regimen disclosed in the 4,273,622 patent, these undesired reactions can be avoided and polymeric components are converted into significant percentages of styrene monomer and other valuable monomers.
U.S. Pat. No. 2,929,855 is concerned with residues obtained as by-products in the preparation of styrene from ethylbenzene by oxidation of ethylbenzene to acetophenone, hydrogenation of acetophenone to alpha-methylbenzyl alcohol and dehydration of alpha-methylbenzyl alcohol to styrene. The treatment method disclosed comprises hydrogenolysis of the residues at 400.degree. C. to 700.degree. C. and pressures above 500 psi. Hydrogen is provided at the rate of 4-10 lbs. per 100 lbs. of feed. Hydrogen feed rates below 2 lbs. per 100 lbs. of residue feed are avoided because of coke formation in the reactor. Preferred hydrogenolysis operating conditions are 500.degree.-600.degree. C. and 2,000-4,000 psi. Products recovered from the process are benzene, toluene and ethylbenzene.
The 4,273,622 and 2,929,855 patents both teach useful methods for the recovery of valuable products from alphamethylbenzyl alcohol dehydration residues. However, neither method has been found entirely satisfactory for commercial application. The principal drawback of the 4,273,622 is the need for a difficult and expensive separation of styrene and ethylbenzene in the product. The principal drawbacks of the 2,929,855 method are the severe operating conditions employed and attended high capital equipment costs.
An object of this invention is to provide an improved process for recovering valuable products from the residual portion of the reaction mixture obtained in the dehydration of alpha-methylbenzyl alcohol. A further object of this invention is a method for the recovery of valuable products from alpha-methylbenzyl alcohol dehydration residue which method is compatable with, and enhances the yields of, liquid-phase dehydration processes of the type described in the Becker patent. Other objects will be apparent from the following description of this invention.